<p>The high computational cost of chemical transport models (CTMs) is a potential bottleneck for the rapid assimilation of ozone (O<sub>3</sub>) observations. Here we developed a single tracer tagged-O<sub>3</sub> mode to build the capability of the GEOS-Chem model for rapid simulation of tropospheric O<sub>3</sub>. The tagged-O<sub>3</sub> mode demonstrates high consistency with GEOS-Chem full-chemistry simulation and dramatic reductions in computational costs by approximately 91–94 %. The tagged-O<sub>3</sub> simulation was combined with China Ministry of Ecology and Environment (MEE) and Ozone Monitoring Instrument (OMI) O<sub>3</sub> observations to investigate the changes in tropospheric O<sub>3</sub> over E. Asia in 2015–2020. The assimilated O<sub>3</sub> concentrations demonstrate good agreement with O<sub>3</sub> observations: surface O<sub>3</sub> concentrations are 42.9, 41.8 and 42.1 ppb; and tropospheric O<sub>3</sub> columns are 37.1, 37.9 and 38.0 DU in the simulations, assimilations and observations, respectively. The assimilations indicate rapid increases in surface O<sub>3</sub> by 1.60 (spring), 1.16 (summer), 1.47 (autumn) and 0.80 (winter) ppb yr<sup>-1</sup> over E. China in 2015–2020, and the increasing trends are underestimated by the a priori simulations. More attention is thus suggested to the rapid increases in O<sub>3</sub> pollution in spring and autumn. Furthermore, we find stronger increases in tropospheric O<sub>3</sub> columns over highly polluted areas, which may reflect the larger contributions of local emissions. The large discrepancy in the trends in tropospheric O<sub>3</sub> columns by assimilating surface and satellite observations further indicates the possible uncertainties in the derived free tropospheric O<sub>3</sub> changes. The rapid O<sub>3</sub> assimilation capability is a useful tool for the extension and interpretation of atmospheric O<sub>3</sub> observations.</p>